Lamellidens marginalis (Bivalvia: Eulamellibranchiata) is a freshwater edible mollusc distributed in the wetland of different districts of West Bengal. The natural habitat of this species is under risk of contamination by multineem, a newly introduced azadirachtin-based (limonoid) pesticide. The hemocyte of Lamellidens marginalis is an immunoeffector cell of hemolymph, capable of discriminating self and nonself surface by immunoadhesion. Glass surface adhesion of hemocytes were screened against 0.03 ppm, 0.06 ppm, and 0.09 ppm of azadirachtin for different spans of exposure. The surface adhesion property of hemocytes decreased against all the concentrations tested. The data is indicative of impairment in immunological response of Lamellidens marginalis in its natural habitat, leading to a decrease in the population size.

Lamellidens marginalis is an important member of the freshwater benthic community. This species is a filter feeder and reduces the sedimentation load of the water column. Lamellidens thrive on plankton and aquatic plants distributed in biosafe and biounsafe environment. Currently, a decline in population of Lamellidens marginalis is being noticed in several wetlands of urban and rural West Bengal. The habitat of Lamellidens marginalis receives diverse toxins of known and unknown chemistry. In recent years, Multineem, an azadirachtin-based biopesticide has been introduced in agriculture for the protection of crop against pest. Highly toxic azadirachtin residues are potent neurotoxins (Schmutter, 1990) and produce adverse effects at various levels of physiological and immunological parameters of aquatic animals. Agricultural runoff loaded with azadirachtin adversely affect the non-target invertebrates of aquatic ecosystem including Lamellidens marginalis. Molluscs are capable of discriminating the self from nonself by discrete population of hemoytes as evident from surface adhesion assay (Cheng and Bayne, 1995). Hemocytes are immunologically competent cells of blood of molluscs and are capable of expressing immunological responses through phagocytosis (Sauve et al., 2002), surface adhesion (Couch et al., 1990), aggregation (Showaza and Suto, 1990), nodule formation (Eatner and Vinson, 1983), etc., under the exposure of toxins and parasites. In this study, adult Lamelliden marginalis exposed to 0.03 ppm, 0.06 ppm, and 0.09 ppm of azadirachtin in batches and nonself surface adhesion of hemocytes of Lamellidens marginalis exposed to azadirachtin were screened in controlled laboratory conditions. Teh surface adhesion efficacy of selected population of hemocytes was determined by estimating the ratio of adherent and nonadherent subpopulation of hemocytes. The alteration of nonself surface adherence of hemocytes is indicative of a shift in immunological responses in Lamellidens marginalis under the sublethal exposure of azadirachtin.

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